Layers of amorphous silicon and germanium have in the past been produced by a variety of methods, for instance, by sputtering, vacuum evaporation, plasma deposition, and chemical vapor deposition (CVD) at approximately atmospheric pressure. These methods typically result in relatively low density material, often less than 90% of the corresponding crystalline density. The material formed is typically also characterized by a substantial density of deep electronic trap sites, for instance dangling bonds, which result in short lifetimes of free carriers. In order for semiconductor material to be of technological interest, the ability to control its electronic properties by means of substitutional doping is generally required. Early attempts at doping of amorphous silicon and germanium, typically of material produced by sputtering or evaporation, showed that these materials were not suitable for controlled doping.
Several years ago. W. E. Spear and P. G. Le Comber, Solid State Communications, Vol. 17, pages 1193-1195 (1975) announced that they had succeeded in substitutionally doping amorphous silicon, and thereby producing changes in conductivity of many orders of magnitude. The material was produced by radio frequency glow discharge decomposition of silane (SiH.sub.4).
It is now known that amorphous silicon or germanium (a-Si or a-Ge) produced by means of a glow discharge, or, more generally, by means of plasma deposition, differs from the corresponding materials produced by either sputtering or evaporation primarily in its chemical composition. In particular, plasma deposited a-Si and a-Ge contain substantial amounts of hydrogen, typically between about 10 and 35 atomic percent. The hydrogen is thought not only to saturate dangling bonds, but also to modify the whole structure of the material, permitting a substantial fraction of dopant atoms present to assume substitutional positions. See, for instance, H. Fritzsche et al, Solid State Technology, January 1978, pages 55-60. Soon after Spear and LeComber's announcement, amorphous p-n and Schottky barrier junctions were fabricated in several laboratories from plasma-deposited low-density, hydrogen-rich a-Si. As was pointed out by Fritzsche (op. cit.), these developments are of great interest because thin film processing of amorphous semiconductor devices permits easy and cheap manufacture of large area devices on a variety of differently shaped or flexible substrates, as might be needed, for instance, for solar energy applications.
Chemical vapor deposition (CVD) is one of the most important methods for depositing thin layers of a large variety of materials, particularly in solid state electronics. The method comprises reacting constituents of a vapor phase to form a solid product on a substrate surface, commonly by means of pyrolysis, i.e., thermal decomposition of one or more precursor compounds.
Amorphous silicon (and a-Ge) can be deposited by CVD, as was indicated above. Typically, this is done by means of decomposition of SiH.sub.4 (or GeH.sub.4) at a substrate temperature less than about 700.degree. C. (&gt;450.degree. C.), the atmosphere comprising, in addition to the appropriate precursor and perhaps a dopant precursor such as PH.sub.3 or B.sub.2 H.sub.6, a diluent gas such as nitrogen, hydrogen or argon, the total pressure of the atmosphere being typically near ambient pressure, in any case greater than about 10 Torr. Deposition of a-Si and a-Ge by CVD can result in low-hydrogen material which can be substitutionally doped to some degree. However, the presence of hydrogen in these materials results in an increased doping efficiency and improved transport properties. See, for instance, M. Hirose et al, Journal of Non-Crystalline Solids, 35 and 36, pages 297-302, (1980). Hydrogen can be incorporated into the amorphous layers either during the deposition process, or during a subsequent treatment, such as plasma hydrogenation or hydrogen ion implantation. Heavily doped material generally has a room temperature D.C. conductivity of less than about 1 (ohm.cm).sup.-1. For instance, P-doped a-Si typically has 0.1 (ohm.cm).sup.-1 for a 1:100 dopant-yielding precursor/Si-yielding precursor ratio in the feed gas for the reaction, with the conductivity typically dropping by some six orders of magnitude for a decrease of dopant ratio by two orders of magnitude. See, for instance, M. Hirose et al, (op. cit.). However, N. Sol et al, ibid, pages 291-296, report a conductivity of nearly 1 (ohm.cm).sup.-1 for their P-doped a-Si.
Amorphous silicon produced by the above methods typically has an optical bandgap considerably smaller than 1.8 eV, the bandgap of maximally dense a-Si. For instance, some CVD a-Si has been reported to have a gap of 1.45 eV. (M. Hirose et al, op. cit., page 94.) The bandgap energy is usually a sensitive function of the void fraction in a-Si, and thus an indicator of density.
Recently, low pressure CVD (LPCVD) has assumed technological importance. This method typically uses pure reactants at reduced pressure, typically less than about 10 Torr. This method has several advantages over CVD, among them economy, since higher packing density of substrates to be coated is possible, resulting in increased throughput and, therefore, lower production cost. Lower cost also results from the elimination of diluent gases. Also, particulate contamination is generally reduced, and film uniformity improved, resulting in higher yields. LPCVD is currently widely used for depositing films of insulators and of polycrystalline silicon in semiconductor device manufacture.